This invention relates to the dynamic positioning of a moving body by means of a magnetic field. The moving body comprises one or more magnetometers providing the components of the magnetic field in the reference mark of this moving body at one or more points of the moving body. The magnetic field is produced by an apparatus connected to the fixed reference marks.
Signals controlling the movement of the moving body are prepared from these magnetic field measurements in order to position the moving body in a specified position.
The invention relates more particularly to ships which are to be immobilised at sea and more particularly alongside a drilling platform. The method consisting of immobilising a ship fitted with propellers correcting the action of the wind, the current and the swell is known as dynamic anchoring.
A known method of dynamic anchoring consists in stretching a cable between the ship and a fixed point on the sea bed and measuring on board the ship the angles of inclination of the cable relative to reference points connected to the ship. From these angular measurements and a measurement of the head of the ship using a gyrocompass, a computer prepares the control signals for the propellers in order to immobilise the ship.
The drawback of this method of dynamic anchoring is that the ship has to be connected to the sea bed by means of a cable.
Another method consists in wetting acoustic beacons fixed to the sea bed and taking up position using the signals emitted by these boundary marks.
This method has the disadvantage of requiring a large infrastructure, which is only justifiable for long-term anchoring. Moreover, the acoustic method is difficult to put into practice owing to environmental noise and the noise of the boat itself.
The same applies to the methods of positioning using radar, which have the particular disadvantage that the signals may be jammed by the presence of other boats also equipped with radar.
Finally, the optical positioning methods have the disadvantage of not being possible in foggy weather and in the presence of smoke.